With reference to FIG. 1, it is known practice to furnish a vehicle 100, that is totally or partially electrically propelled, with a power converter 102 capable of raising the voltage of a battery 104 in order to power an electrical machine 106, typically via inverters 108.
Because of the powers at work in such a converter 102, usually between 20 and 1000 kW, it may be worthwhile to make a multi-cell converter in which a power-supply current originating from the battery 104 is distributed amongst multiple conversion cells.
The use of such a multi-cell converter faces a problem of cost notably because of the quantity of silicon required by switches 111 and 113 used in a cell, and notably in their transistors 103 or 105 and their power diodes 107 or 109.
In the case of electrical vehicles and more particularly of hybrid vehicles, the volume and weight of these cells are also important criteria and, notably, the usually considerable size of the inductive elements—a coil 110 in this example—becomes problematic.
Efficiency is another important criterion for the use of a converter since the latter has a direct influence on the range of the vehicle 100.
In order to increase this efficiency, it is known practice to use inversion cycles of the current flowing in the inductive element 110 of the converter in order to use a switching method called ZVS, for “Zero Voltage Switching”, as described below with reference to FIGS. 2 to 5.
More precisely, FIG. 2 illustrates such an opening of the switch 113 of the converter 102 under a practically zero voltage obtained by virtue of a capacitor 204 since, as described below, this capacitor 204 is discharged during this opening.
Although the current 302 (FIG. 3) flowing in the collector of the transistor 105 is not totally zero when the voltage 304 at the terminals of the said transistor 105 (collector/emitter voltage) begins to increase, this method makes it possible practically to greatly reduce the losses associated with an opening of the switch 105. In the example of FIG. 3, the orders of magnitude at work are indicated as a key in the figure.
Consequently, the capacitor 204 can be discharged by inverting the current i passing through the inductive element 110 such that, with the capacitor 204 being thus discharged, the switch can again close under zero voltage.
With reference to FIG. 4, the converter 102 is shown during such an inversion of the current i passing through the inductive element 110 of the cell, that is to say at the opening of the switch 111. Since the flow of current through the switches 111 and 113 is blocked, the capacitor 204 discharges, and then, when it is completely discharged, it places the diode 109 of the switch 113 in conduction.
In this case, the voltage 504 (FIG. 5) at the terminals of the switch 113 decreases rapidly during the discharging of the capacitor (step 510) and then becomes negative such that the diode 109 (step 512) is on-state.
Consequently, the current 502 in the inductive element 110 becomes positive again and the cycle is repeated with an opening of the switch 113 as described above.
Although the use of this ZVS method with a fixed switching frequency may be satisfactory when the converter is operating at high load, it seems that this ZVS technique does not allow satisfactory efficiency at low load.
Specifically, it seems problematic that the current is greatly modulated irrespective of the load, whether it be at low load, that is to say when the average current is close to zero (FIG. 7) or in the case of a high load, that is to say when the average current is, for example, close to 50 A (FIG. 6). In this example, the inversion is of the order of 100 A peak-to-peak, which generates considerable iron losses in the inductor.
In order to alleviate this problem, it is known practice to use the ZVS method described above in a mode called critical conduction. In such a mode, the inversion of the current is forced for a time that is relatively short but sufficient to discharge the capacitor used to zero the voltage of the switch, as shown in FIG. 8.
In this case, the current is controlled by a first threshold which regulates the average value of the current and by a second threshold, with an opposite sign, which discharges the said capacitor, the alternation between these two thresholds being subjected to a variable frequency.
Problematically, it seems that this critical conduction mode generates current inversions in the inductive elements which make it difficult to design the latter. In truth, the amplitude of the current inversions may be greater than 100% of the value of the peak current over a high-frequency range (between 20 kHz and 80 kHz for example) which makes the power losses in the inductor unacceptable in terms of temperature rise and efficiency.
Finally, it should be pointed out that the inductive elements are usually manufactured based on materials of the ferrite or nanocrystalline type because of the resistivity of the material for the first and the thinness of the strips forming the core for the second, and because of their common capability to limit the generation of eddy currents and therefore to limit the losses.
Unfortunately, ferrite is a material which saturates with a relatively weak magnetic field relative to other, iron-based, magnetic materials.
The consequence is the considerable magnetic volume required to manufacture the inductive element, which may be unacceptable in a hybrid-vehicle application, or the limited power storage in these materials because of their very great permeability.
In terms of power storage, iron-based and silicon-based materials could be more suited to vehicle converters because their saturation threshold can sometimes exceed 2 Teslas. In addition, these materials are very widely used in power conveyance or conversion (transformer, generator, electric motor, etc.), usually in the form of laminated metal sheets.
Problematically, these materials have a high level of losses at high frequency because of the modulations of flux generated by these high frequencies. This is why the frequencies used in power conveyance vary across a relatively low range of frequencies, typically between 50 Hz and 1 kHz, with switching frequencies rarely exceeding approximately 10 kilohertz.